Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack. It is generally convenient to locate all of the inlet and outlet ports at the same end of the stack.
In conventional electrochemical fuel cell stacks employing solid polymer ion exchange membranes, the manifolds for directing reactants and products to and from the individual fuel cells are formed by aligning a series of manifold openings or perforations formed at the interior of the reactant flow field plates. For example, Watkins et al. U.S. Pat. No. 5,108,849 discloses, in FIG. 4 and the accompanying text, a reactant fluid flow field plate having a plurality of openings formed at the corners, including a fluid supply opening and a fluid exhaust opening. Each channel formed in the Watkins flow field plate includes an inlet end directly connected to the fluid supply opening and an outlet end directly connected to the fluid exhaust opening. The channels direct the reactant gas stream from the supply opening to the central, electrocatalytically active area of the fuel cell. When multiple fluid flow field plates are arranged in a stack, each of fluid supply and exhaust openings aligns with the corresponding opening in the adjacent plates to form a manifold for directing the reactant fluid stream through the extent of the stack.
In other types of conventional fuel cell stacks, primarily those employing liquid electrolytes, the manifolds for directing reactants and products to and from the individual fuel cells are located in a frame surrounding the cell plates. For example, in Uline U.S. Pat. No. 3,278,336, a frame having apertures formed in its upper and lower marginal portions introduces reactant gas and electrolyte to the electrode and discharges reactant gas and electrolyte from the electrode.
Erickson U.S. Pat. No. 3,615,838, Warszawski U.S. Pat. No. 3,814,631, Bellows U.S. Pat. No. 4,346,150, Alfenaar U.S. Pat. No. 4,403,018, Romanowski U.S. Pat. No. 4,743,518, and Okada U.S. Pat. No. 4,943,495 disclose additional examples of conventional fuel cell stacks in which the manifolds for directing reactants and products to and from the individual fuel cells are located in a frame surrounding the cell plates. Frame manifold structures have inherent disadvantages in that (1) frame manifold structures increase the overall volume of the fuel cell stack, (2) frame manifold structures are generally expensive to manufacture and/or mold, (3) frame manifold structures generally employ complicated and potentially inefficient sealing schemes to isolate the reactant and electrolyte streams from each other, from the electrochemically active region of the fuel cell, and from the external environment, and (4) frame manifold structures impede access to the interior stack components, such as the fuel cells themselves and associated structures such as bus plates.